Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One such user interface device is a touch panel having multiple buttons for controlling a device, such as a touch panel of a television (TV), a video cassette recorder (VCR), a digital video recorder (DVR), a digital video disc (DVD) player, a receiver, a computer, a radio, lights, fans, industrial equipment, or the like. For example, considering a standard consumer electronics device like a DVD player, the DVD player front panel has buttons that form a user interface (UI). In general terms, a UI receives input from the user and allows the user to interact with an electronic device. Some UIs use traditional mechanical buttons and some newer UIs use touch-sensor buttons (e.g., capacitance sensing buttons).
Capacitance sensing is used in wide variety of user interface applications. Examples include touchpads on notebook computers, touchscreens, slider controls used for menu navigation in cellular phones, personal music players, and other hand held electronic devices. One type of capacitance touch-sensor device operates by way of capacitance sensing utilizing capacitive sensors. One way in which the capacitance detected by a capacitive sensor changes is as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user's finger. The touch-sensor devices may include single sensor elements or elements arranged in multiple dimensions for detecting a presence of the conductive object on the touch-sensor device. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of activation, position, or the like of the conductive object in relation to the touch-sensor device, such as a touch panel. A touch-sensor strip, slider, or touchpad operates on the same capacitance-sensing principle.
Capacitance sensing has been implemented in a wide variety of electronic devices to replace mechanical buttons in the electronic devices. Capacitance sensing has many advantages over conventional cursor control devices, mechanical switches, and rotary encoders. A principal such advantage is the lack of moving parts, which allows capacitance sensing to provide great improvements in reliability since there are no moving parts to wear out.
Typically, a capacitance sensing system detects changes in capacitance between a sensing element and electrical ground. For example, in a capacitance sensing button, when the users' finger is in close proximity to the sensor element, a capacitance is formed between the sensor element and the finger, and as the finger is effectively at a ground potential, a detectable capacitance to ground is present when the user's finger is close to the sensor element. In a touchpad or slider, the position of the user's finger is detected by measuring the difference in capacitance to ground between a number of sensing elements arranged as an array. The capacitance sensing system detects the presence of the user's finger over a button area and registers a button press as a result.
One type of conventional capacitance sensing touch panel 100, as illustrated in FIG. 1, includes eight touch-sensor buttons 101, which each include a sensor element 102. Surrounding each sensor element 102 is a dielectric 103 (e.g., dielectric material or air) that separates the touch-sensor buttons 101 from a ground conductor 104 that is disposed to surround the sensor elements 102. The change in capacitance on the sensor element 102 resulting from the introduction of either a conductive object into close proximity to the sensor element 102 is sensed. Electric field lines which run between conductive elements, namely the ground conductor 104 and the sensor element 102, through the dielectric 103 and the dielectric above and below the touch panel 100. When nothing changes (e.g., the configuration is static), a given sensor element 102 has a fixed capacitance determined by the properties of the conductor, the dielectric, geometry of the assembly, environmental conditions, as well as other factors. Introduction of a conductive object into close proximity to the sensor element 102 changes the environment, which consequently changes the capacitance of the sensor element 102. For example, placing a finger in proximity to the sensor element 102 raises the capacitance on sensor element 102. The increase in capacitance is registered by the capacitance sensing device as a “button press.”
In almost all button-press applications of capacitance sensing, the conductive elements (e.g., sensor elements 102 and ground conductor 104) are isolated from the user by a dielectric material as an overlay. Since the electric field penetrates the overlay, when the user's finger (or other conductive object) is in proximity to the sensor element 102, the capacitance sensing device can detect a change in the capacitance. It should be noted that since the conductive elements are isolated by an overlay, the sensor elements 102 are not actually touched by the conductive object. As such, sensor elements, such as sensor elements 102, are considered to be proximity sense technology. However, in the proximity sense technology, the sensor element 102 may be a “capacitive proximity sensor” or a “capacitive button sensor” (e.g., touch-sensor button). Whether a sensor element 102 is considered to be a capacitive proximity sensor or a touch-sensor button is based on the sensitivity of the processing device, which processes the data from sensor element 102. The difference between a capacitive proximity sensor and a capacitive button sensor is that the proximity sensor is tuned to respond before the user touches the dielectric overlay and the button sensor is tuned to respond when the user actually touches the surface of the dielectric overlay. As a result, a proximity sensor is usually tuned to be much more sensitive to a change in capacitance than capacitive button sensors. For technical reasons, standard engineering practice for capacitive button sensors is to use a “ground” conductor 104 that surrounds the capacitive button 101.
Conventional UIs that implement both proximity sensing and button-activation sensing use a dedicated, discrete proximity sensor. The proximity sensor element is physically separate from the sensor elements required for capacitive button sensing. For example, the touch panel 100, which includes both proximity sensing and button-activation sensing, requires an additional sensor element 106 to perform proximity sensing, as illustrated in FIG. 1B. The additional proximity sensor element 106 is disposed around the perimeter of the touch panel 100, and is separated from the ground conductor 104 by a dielectric 105. The capacitive proximity sensor 106 is usually large due to the use model, since the desired use model for a proximity sensor is to detect when the user's hand is in proximity to the area of the user interface (e.g., touch panel 100), as opposed to touching the overlay of one of the touch sensor buttons. The capacitive proximity sensor 106 usually surrounds the user interface.